Clocks run fast where gravity is weak

Clocks run fast where gravity is weak

Ten years later, Einstein figured out one more thing: the stronger gravity is, the slower clocks run. Picture Earth’s gravity as a net sagging in the middle. The ground sits deep in the dip, and the satellites sit up where it’s shallow. Gravity at the satellites’ height is about 17 times weaker than on the ground. So a satellite clock runs ahead of ground clocks by about 45.7 microseconds a day.

Why?For the same reason, a clock on a high mountain runs a tiny bit faster than one at the beach. With very precise atomic clocks, you can measure the difference just by raising a clock about 30 cm.

Learn moreThis is called “general relativity.” It was published in 1915. Near a black hole, where gravity is enormous, time slows down far more. You can see that in “Spacetime and Black Holes” from this series. The net is a picture of gravity’s strength, not its real shape.

Time gained each day because of gravity

+45.7 μs

textbook formulas